Polycarbonates and other polymer materials are utilized in optical data storage media, such as compact disks. In optical data storage media, it is critical that polycarbonate resins have good performance characteristics such as transparency, low water affinity, good processibility, good heat resistance and low birefringence. High birefringence is particularly undesirable in high density optical data storage media.
Improvements in optical data storage media, including increased data storage density, are highly desirable, and achievement of such improvements is expected to improve well established and new computer technology such as read only, write once, rewritable, digital versatile and magneto-optical (MO) disks.
In the case of CD-ROM technology, the information to be read is imprinted directly into a moldable, transparent plastic material, such as bisphenol A (BPA) polycarbonate. The information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective metallic film, and the digital information, represented by the position and length of the pits, is read optically with a focused low power (5 mW) laser beam. The user can only extract information (digital data) from the disk without changing or adding any data. Thus, it is possible to "read" but not to "write" or "erase" information.
The operating principle in a WORM drive is to use a focused laser beam (20-40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: "hole burning" is the removal of material, typically a thin film of tellurium, by evaporation, melting or spalling (sometimes referred to as laser ablation); bubble or pit formation involves deformation of the surface, usually of a polymer overcoat of a metal reflector.
Although the CD-ROM and WORM formats have been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable media for optical storage (EODs). There are two types of EODs: phase change (PC) and magneto-optic (MO). In MO storage, a bit of information is stored as a .about.1 .mu.m diameter magnetic domain, which has its magnetization either up or down. The information can be read by monitoring the rotation of the plane polarization of light reflected from the surface of the magnetic film. This rotation, called the Magneto-Optic Kerr Effect (MOKE) is typically less than 0.5 degrees. The materials for MO storage are generally amorphous alloys of the rare earth and transition metals.
Amorphous materials have a distinct advantage in MO storage as they do not suffer from "grain noise", spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film. Bits are written by heating above the Curie point, T.sub.C, and cooling in the presence of a magnetic field, a process known as thermomagnetic writing. In the phase-change material, information is stored in regions that are different phases, typically amorphous and crystalline. These films are usually alloys or compounds of tellurium which can be quenched into the amorphous state by melting and rapidly cooling. The film is initially crystallized by heating it above the crystallization temperature. In most of these materials, the crystallization temperature is close to the glass transition temperature. When the film is heated with a short, high power focused laser pulse, the film can be melted and quenched to the amorphous state. The amorphized spot can represent a digital "1" or a bit of information. The information is read by scanning it with the same laser, set at a lower power, and monitoring the reflectivity.
In the case of WORM and EOD technology, the recording layer is separated from the environment by a transparent, non-interfering shielding layer. Materials selected for such "read through" optical data storage applications must have outstanding physical properties, such as moldability, ductility, a level of robustness compatible with popular use, resistance to deformation when exposed to high heat or high humidity, either alone or in combination. The materials should also interfere minimally with the passage of laser light through the medium when information is being retrieved from or added to the storage device.
As data storage densities are increased in optical data storage media to accommodate newer technologies, such as digital versatile disks (DVD) and higher density data disks for short or long term data archives, the design requirements for the transparent plastic component of the optical data storage devices have become increasingly stringent. In many of these applications, previously employed polycarbonate materials, such as BPA polycarbonate materials, are inadequate. Materials displaying lower birefringence at current, and in the future progressively shorter "reading and writing" wavelengths have been the object of intense efforts in the field of optical data storage devices.
Low birefringence alone will not satisfy all of the design requirements for the use of a material in optical data storage media; high transparency, heat resistance, low water absorption, ductility, high purity and few inhomogeneities or particulates are also required. Currently employed materials are found to be lacking in one or more of these characteristics, and new materials are required in order to achieve higher data storage densities in optical data storage media. In addition, new materials possessing improved optical properties are anticipated to be of general utility in the production of other optical articles, such as lenses, gratings, beam splitters and the like.
Birefringence in an article molded from polymeric material is related to orientation and deformation of its constituent polymer chains. Birefringence has several sources, including the structure and physical properties of the polymer material, the degree of molecular orientation in the polymer material and thermal stresses in the processed polymer material. For example, the birefringence of a molded optical article is determined, in part, by the molecular structure of its constituent polymer and the processing conditions, such as the forces applied during mold filling and cooling, used in its fabrication which can create thermal stresses and orientation of the polymer chains.
The observed birefringence of a disk is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence and the birefringence introduced upon molding articles, such as optical disks. The observed birefringence of an optical disk is typically quantified using a measurement termed "vertical birefringence" or VBR, which is described more fully below.
Two useful gauges of the suitability of a material for use as a molded optical article, such as a molded optical data storage disk, are the material's stress optical coefficient in the melt (C.sub.m) and its stress optical coefficient in the glassy state (C.sub.g), respectively. The relationship between C.sub.m, C.sub.g and birefringence may be expresses as follows: EQU .DELTA.n=C.sub.m .times..DELTA..sigma..sub.m (1) EQU .DELTA.n=C.sub.g .times..DELTA..sigma..sub.g (2)
where .DELTA.n is the measured birefringence and .DELTA..sigma..sub.m and .DELTA..sigma..sub.g are the applied stresses in the melt and glassy states, respectively. The stress optical coefficients C.sub.m and C.sub.g are a measure of the susceptibility of a material to birefringence induced as a result of orientation and deformation occurring during mold filling and stresses generated as the molded article cools.
The stress optical coefficients C.sub.m and C.sub.g are useful as general material screening tools and may also be used to predict the vertical birefringence (VBR) of a molded article, a quantity critical to the successful use of a given material in a molded optical article. For a molded optical disk, the VBR is defined as: EQU VBR=(n.sub.r -n.sub.z)=.DELTA.n.sub.rz (3)
where n.sub.r and n.sub.z are the refractive indices along the r an z cylindrical axes of the disk; n.sub.r is the index of refraction seen by a light beam polarized along the radial direction, and n.sub.z is the index of refraction for light polarized perpendicular to the plane of the disk. The VBR governs the defocusing margin, and reduction of VBR will lead to alleviation of problems which are not correctable mechanically.
In the search for improved materials for use in optical articles, Cm and Cg are especially useful since they require minimal amounts of material and are relatively insensitive to uncontrolled measurement parameters or sample preparation methods, whereas measurement of VBR requires significantly larger amounts of material and is dependent upon the molding conditions. In general, it has been found that materials possessing low values of C.sub.g and C.sub.m show enhanced performance characteristics, for example VBR, in optical data storage applications relative to materials having higher values of C.sub.g and C.sub.m. Therefore, in efforts aimed at developing improved optical quality, widespread use of C.sub.g and C.sub.m measurements is made in order to rank potential candidates for such applications and to compare them with previously discovered materials.
In applications requiring higher storage density, the properties of low birefringence and low water absorption in the polymer material from which the optical article is fabricated become even more critical. In order to achieve higher data storage density, low birefringence is necessary so as to minimally interfere with the laser beam as it passes through the optical article, for example a compact disk.
Another critical property needed for high data storage density applications is disk flatness. It is known that excessive moisture absorption results in disk skewing which in turn leads to reduced reliability. Since the bulk of the disk is comprised of the polymer material, the flatness of the disk depends on the low water absorption of the polymeric material. In order to produce high quality disks through injection molding, the polymer, such as polycarbonate should be easily processed.
U.S. Pat. No. 4,950,731 discloses polycarbonate materials or use in optical materials derived from bisphenol A and SBI. Bisphenol A polycarbonate has high stress optical coefficient in the melt phase (C.sub.m) and a high stress optical coefficient in the glassy state (C.sub.g) which is moderated by the incorporation of 6,6'-dihydroxy-3,3,3',3'-tetramethylspirobiindane (SBI), the homopolycarbonate of which has a negative C.sub.m value. Copolycarbonates prepared from BPA and SBI have been shown to possess stress optical coefficients in the melt (C.sub.m) of zero or near zero. Such copolymers lack the processibility of bisphenol A polycarbonate, however, and have higher affinity for water.
U.S. Pat. No. 5,132,154 discloses polycarbonate mixtures in optical applications. The polycarbonate resin comprises units which contain a bisphenol having an alicyclic ring structure, which is substituted by alkyl groups on at least one member of the ring structure. Such polymers are not the subject of this invention.
Japanese Kokai Patent Application No. 3-237130 discloses a polyether resin for use in optical materials. The disclosure relates to polyether resin and not polycarbonate resin, see page 4, last line.
Japanese Kokai Patent Application No. 4-41524 discloses a polyester carbonate for use in optical materials. The polyester carbonate resin comprises units derived from phthalic acid. Such polymers are not the subject of this invention.
Japanese Kokai Patent Application No. 3-221523 discloses a polyformal resin for use in optical articles obtained by the reaction of a divalent phenolic compound and a dihalogen compound. The reference does not relate to polycarbonate resins as disclosed on page 4, and characterizes polycarbonate resin as exhibiting high birefringence, as disclosed on page 4, lines 8-11.
Japanese Kokai Patent Application No. 3-162413 discloses a polymer for use in optical materials. The polymer comprises residues of spirobichroman. Such polymers are not the subject of this invention.
Japanese Kokai Patent Application No. 10-176046 discloses a polycarbonate copolymer for use in optical articles comprising residues of 2-2-bis(3-tert-butyl-4-hydroxy-6-methylphenyl)-n-butane. Such polymers are not the subject of this invention.
Japanese Kokai Patent Application No. 4-345616 discloses a polycarbonate which may be used in optical recording media, the polycarbonate comprising BPA or other aromatic hydroxy compounds and SBI.
Japanese Kokai Patent Application No. 9-6023 discloses a copolymer binder with abrasion resistance and cracking resistance for use in photoresists. There is no disclosure of optical materials having birefringence and other properties suitable for use in optical articles, or optical articles made from these materials.
Examined Japanese Patent No. 06-52585 discloses a copolymer which may be used in optical materials. The starting materials may include bisphenols such as 1,1-bis(4'-hydroxyphenyl)cyclohexane.
U.S. Pat. No. 5,858,833 and EP 0846711 disclose optical disk grade copolyestercarbonates derived from hydroxyphenylindanols, having units derived from 6-hydroxy-1-(4'-hydroxyphenyl)-1,3,3-trimethylindane (CD-1).
U.S. Pat. No. 4,304,899 and EP 0016167 disclose polycarbonate compositions having improved barrier properties. The disclosures do not teach compositions for use in optical media.
U.S. Pat. No. 5,424,389 discloses copolymers of bisphenol A and SBI for use in optical applications. Such copolymers are not the subject of this invention.
U.S. Pat. No. 5,633,060 discloses an optical disk substrate derived from 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane (BPI). Additionally the polycarbonate may comprise units derived from 4,4'-(m-phenylenediisopropylidene)diphenol and/or 2,2-bis(3-methyl-4-hydroxyphenyl)propane. Polycarbonates based on BPI are not the subject of this invention.
There exists a need for compositions having good optical properties and good processibility and which are suitable for use in high density optical recording media. Polycarbonates manufactured by copolymerizing the aforementioned aromatic dihydroxy compounds, such as bisphenol A, with other monomers, such as SBI, may produce acceptable birefringence; however the glass transition temperature is often too high, resulting in poor processing characteristics. Consequently, the obtained moldings have low impact resistance. Further, the water absorption of such polycarbonates is unacceptable for higher density applications.